326 ON THE CHANGE OF KEFRANGIBILITY OF LIGHT.
this way fixed lines may be seen on common white paper far beyond H. These lines may be seen without the use of the blue glass, by allowing the bright colours to pass by the edge of the paper, and receiving on it only the extreme violet and invisible rays.
95. Paper coloured by turmeric having exhibited so well the sensibility of that substance, I was induced to try various other washed papers, in fact, papers washed with most of the fluids with which I had made experiments. I found almost always that sensitive solutions gave rise to sensitive papers, exhibiting a change of refrangibility of the same character as that shown by the solution. Besides the turmeric paper, the two most remarkable were paper washed with a pretty strong solution of sulphate of quinine, and paper washed with the extract from the seeds of the Datura stramonium. I should here observe, that it was not till long after the time when these experiments were made that I was acquainted with the high sensibility of a decoction of the' bark of the horse-chestnut. The former of the papers just mentioned exhibited the fixed lines of the invisible rays on !'• a blue, and the latter on a green ground. The dispersion pro-
duced by the quinine paper was not exhibited so early in the spectrum as in the case of turmeric, nor was it so copious in the extreme violet rays, and for some distance further on, but the quinine paper seemed superior to the other for showing the fixed lines of extreme refrangibility. With the turmeric paper the group n was plain enough, but with the quinine paper I have seen some fixed lines of the group p. The stramonium paper was, on the whole, I think, superior to the quinine paper in point of the copiousness of the dispersed light, but seemed hardly equal to it for showing the fixed lines of extreme refrangibility. However, it is likely that paper washed with a solution of the sensitive principle in a state of purity would have been quite equal to the quinine paper in this respect.
! 96. A washed paper is a little more convenient for use than
a solution, but, as might be expected, it does not show the fixed lines with quite as much delicacy, nor is it quite so good for tracing the spectrum to the utmost limits to which it can be traced with the substance employed.